The use of solvent extraction to separate heavy hydrocarbon materials such as, for example, steam and vacuum reduced crudes into two or more of their component parts is well known. In this regard, many different solvent extraction processes have been proposed or are in use for performing the separation. One widely employed solvent extraction process, utilizing a vertically positioned extraction vessel, is the relatively low temperature, countercurrent flow, solvent deasphalting process described in R. A. Meyers' Handbook of Petroleum Refining Processes, Part 8.1, pp 19-51, McGraw-Hill Book Co., N.Y., N.Y. (1986). Typically, this process entails diluting a heavy hydrocarbon material or feedstock with a quantity of an extraction solvent, adjusting the diluted feedstock to the desired extraction temperature and introducing the diluted feedstock into a medial section of the extraction vessel. Contemporaneously with the introduction of the diluted feedstock, further extraction solvent is introduced into a bottom section of the extraction vessel whereby the feedstock and extraction solvent undergo intimate contact while flowing in countercurrent directions. This contact results in the lower molecular weight components contained in the feedstock being extracted therefrom and in the formation of separate and distinct extract and raffinate phases. The extract phase thus formed contains the lower molecular weight hydrocarbon components of the feedstock (which components comprise a so-called deasphalted oil) and the major portion of the solvent while the raffinate phase contains the remaining higher molecular weight hydrocarbon components of the feedstock, (including the high molecular weight asphaltenes and intermediate molecular weight resins) as well as the Conradson carbon precursors and the bulk of the heavy metals contained therein and a residual portion of the solvent. Following the recovery of these phases, each then is subjected to further processing to strip and individually recover the solvent portions, the deasphalted oil and the higher molecular hydrocarbon components (the latter containing both the Conradson precursors and heavy metals).
In the practice of the above generally described solvent deasphalting process, it is conventional practice to control the quantity and quality of the deasphalted oil extracted from the feedstock either by increasing or decreasing the density of the extraction solvent. On a day-to-day operating basis, this change in density usually is accomplished by varying the temperature of the feedstock and solvent being introduced into the extraction vessel although changes in operating pressure also can be employed.
It also is possible to provide for significant improvements in the quality of the deasphalted oil product for any given desired yield of deasphalted oil product by further maintaining a temperature gradient across the extraction zone. This practice typically involves maintaining a higher temperature in the top or rectification section of the extraction vessel, by means of steam coils fitted therein, and a lower temperature in the bottom or stripping section of the extraction vessel. This temperature gradient generates an internal reflux due to the lower solubility of the heavier hydrocarbon components of the feedstock in the solvent at the higher temperatures in the top section of the extraction vessel compared to their higher solubility in the solvent at the lower temperatures in the bottom section of the extraction vessel. While the use of temperature induced internal reflux within an extraction vessel does, in fact, improve the quality of the deasphalted oil products, such use results in a significant increase in utilities consumption, particularly steam. Thus, the development of means for maintaining or even improving the quality of the deasphalted oil products produced by such processes without adversely affecting the utilities consumption of such processes would be a significant contribution to this field.